The mechanisms by which polypeptide hormones and cAMP regulate protein biosynthesis will be studied using established functional lines of rat pituitary tumor cells (GH3). The regulation of prolactin and growth hormone biosynthesis by thyrotropin-releasing hormone (TRH) and other hormones, and by cyclic AMP derivatives will be further defined, using specific DNA probes complementary to prolactin and growth hormone mRNAs. The relationship of cell growth to regulation of prolactin gene expression will be evaluated. The site(s) at which TRH and other hormones act to increase intracellular levels of prolactin mRNA will be defined to distinguish between regulation of the levels of transcription, mRNA processing or transport, mRNA catabolism, or translation. The putative nuclear RNA precursors of the prolactin mRNA will be characterized, and the organization of the prolactin gene will be defined. Regulation of another gene product, glutamine synthetase, will be defined using a "specific" cDNA probe, and the possible expression of the calcitonin gene in these cells and in the normal rat pituitary gland will be evaluated. The possible regulation of mRNA translation will be further studied by analysis of the function of the 48,000 dalton mRNA-associated protein and of the functional significance of cAMP-dependent phosphorylation of several protein components of the translational apparatus on the rate of initiation and peptide chain elongation of total and specific mRNAs under hormomal control.